Investment casting, also referred to as the lost wax process, is a casting process particularly suited for the production of small metal parts with extreme dimensional accuracy. The investment casting process is widely used for the fabrication of turbine and stator blades for gas turbine engines. Blades produced by this process have the advantage of requiring only minimal machining following casting. This process is discussed in U.S. Pat. Nos. to Earl, 1,831,555; Watts, 3,590,905; Horton, 3,686,006 and Moren, 3,179,523 and 3,196,505.
Turbine efficiency is closely related to operating temperature. Demands for improved efficiency have resulted in the development of more heat resistant alloys. These alloys are generally characterized by containing quantities of highly reactive alloy additions. The development of such improved alloys has required concurrent improvements in mold materials so as to reduce the interaction between the casting alloy and the mold surface. This type of interaction is highly undesirable since it results in surface defects in the cast product which can lead to failure either through corrosion or mechanical fatigue.
Another technique which has been employed to improve the high temperature properties of superalloys is directional solidification. In this technique a molten casting is slowly solidified at a controlled rate so that the interface between the molten and solidified alloy passes slowly along the longitudinal axis of the part. One result of this technique may be to produce a series of columnar grains with the longitudinal axis of the grains being oriented with the longitudinal axis of the casting. Improved longitudinal high temperature properties are obtained as a result of the reduction in grain boundary area perpendicular to the longitudinal axis. This technique is described in the VerSnyder U.S. Pat. No. 3,260,505. The solidification rates used in the directional solidification process can be relatively low, on the order of 0.1 to 1 inch per hour. Accordingly, substantial time periods of up to 12 hours may be required for the total solidification of a part produced by this process. The mold material adjacent to that part of the directionally solidified casting which solidifies last may therefore be exposed to the molten material for time periods of up to 12 hours. For this reason it has been found that many conventional mold materials which in the past have been found wholly satisfactory for nickel and cobalt superalloys, do not provide adequate performance when employed in the directional solidification process, particularly with some of the more advanced superalloys like those of the family of directionally solidified eutectics. Accordingly, it is the purpose of the present invention to describe a mold material and fabrication technique suited for the production of directionally solidified nickel and cobalt castings and other high temperature alloys.